Biosensing instruments for detecting analyte concentrations (e.g. glucose and cholesterol) are known in the prior art. Recently, an amperometric biosensor has appeared for measuring glucose concentrations in blood samples. In European Patent Application 0 230 472 to Nankai et al. and in PCT Published Application WO 89/08713 to Pottgen et al., amperometric techniques for determining glucose concentration are disclosed. Each system is dependent upon a reaction wherein glucose, in the presence of an enzyme, e.g., glucose oxidase, catalyzes a reaction of potassium ferricyanide to potassium ferrocyanide. After that reaction has completed, a voltage applied across the reaction zone causes the reaction to reverse with an accompanying generation of a small, but measurable, current. That current is termed the Cottrell current and, in dependence upon the concentration of glucose in the reaction zone, will follow a predetermined curve during the reverse reaction. A reading of the Cottrell current can then be converted into an indication of glucose concentration.
A number of problems exist in amperometric biosensors which employ the Cottrell current to provide an indication of an analyte concentration. As above indicated, the forward reaction must initially be allowed to proceed to completion before being reversed by an application of a voltage to electrodes which span the reaction zone. Thus, provision must be made to enable the on/off switching of a known excitation potential across the reaction zone. Without a known voltage being applied, current reading accuracies cannot be assured. Furthermore, since biosensors for determining the concentration of analytes such as glucose and cholesterol are often destined for home use, provision must be made for unskilled use of the instrument.
Accordingly, it is an object of this invention to provide a voltage excitation circuit for an amperometric biosensor which enables a potential applied to the biosensing cell to be accurately determined.
It is a further object of this invention to provide a biosensing excitation circuit which provides a clear indication when a test cell is either improperly inserted or not inserted in the biosensing instrument.